The semiconductor industry has experienced rapid growth due to improvements in the integration density of a variety of electronic components (e.g., transistors, diodes, resistors, capacitors, etc.). For the most part, this improvement in integration density has come from shrinking the semiconductor process node (e.g., shrinking the process node towards the sub-20 nm node). Commensurate with shrunken dimensions is a move to lower operating voltages with each successively shrunken node.
Integrated circuits interface with other integrated circuits through input/output (I/O) circuitry. Not all integrated circuits use the same voltage conditions, however. In modern processes, for example, integrated circuits commonly operate at any of 1.8 Volts, 2.5 Volts, and 3.3 Volts. Thus, a 1.8 Volt integrated circuit may boost its voltage output to interface with a 2.5 Volt or 3.3 Volt integrated circuit, and a 3.3 Volt integrated circuit may drop its voltage output to interface with a 2.5 Volt or 1.8 Volt integrated circuit.
Not only do integrated circuits operate at different voltages, but integrated devices within the integrated circuits may also operate at different voltages. Thus, a 1.8 Volt integrated circuit may include core logic devices that operate at 0.9 Volts. The core logic devices are typically faster, and consume less power than standard devices in the 1.8 Volt integrated circuit. Although the core logic devices are faster and consume less power, they are also more fragile, having lower tolerance for high voltage biasing. I/O devices typically operate at higher voltages, such as 3.3 Volts and 1.8 Volts, but have drawbacks of higher power consumption and slower speed. As a result, when an integrated circuit using mainly core logic devices for signal processing is required to interface with a legacy integrated circuit, or an integrated circuit using an older process node (and thus higher nominal operating voltage), I/O devices are used as an interface between the core logic devices and the legacy integrated circuit. Understandably, conversion from the core logic device voltage (0.9 Volts) to the I/O device voltage (1.8 Volts) is required to output the relatively lower voltage signals from the core logic devices to the I/O devices, and another conversion is required to output the voltage signals at the I/O device voltage (1.8 Volts) to the interfaced integrated circuit which operates at a higher voltage (2.5 Volts or 3.3 Volts). Input buffers are used, among other things, to drop higher operating voltages to lower operating voltages, e.g. 3.3 Volts to 0.9 Volts.